The present invention relates to a digital signal repeating system as between communication cables in a communication system in which data transmission is effected in a time division multiplex mode using the communication cables.
As computers have proliferated and as digital signal processing techniques have progressed, data communication techniques have moved into the limelight, by which techniques a communication system and a data processing system are combined so as to enable information to be processed on-line. A small-scale communication system, such as a private communication system installed on the precincts of government and public agencies, companies, or the like, and especially such as a communication system operating in packet form using a communication cable, such as a coaxial cable, has attracted public attention in view of its inexpensiveness, high reliability and high transmission efficiency.
In such a packet form communication system, a communication cable for effecting bidirectional transmission is disposed in a laboratory, office or the like. A number of personal stations are connected so that messages, each divided into data blocks of 1,000 to 2,000 bits, may be transmitted from each station. Each message is additionally provided with a header such as its destination, running number, or the like. In this communication system, control functions are completely distributed to the respective stations and therefore the network per se is a mere passive transmitting medium having no control functions. Accordingly, each station begins to transmit a message after it has confirmed emptiness of the tranmission line. When a collision between one station's packet signal and another packet signal from another station occurs during the transmitting operation, both concerned stations stop their transmitting operations. Each of the stations which has stopped its transmitting operation will then try to transmit the message again after the lapse of a random queuing time.
In such a communication system, any user at any station not only can access one and the same computer but also can utilize any hardware such as a memory means or any software such as programs, numbers of hardware units and softwares distributed to plural stations. That is, in this communication system, devices such as high speed or high precision printers, large scale files, or the like, which have been concentrated at the location of a large-sized central computer in a time sharing system, may be utilized under the condition that they are distributed to the respective stations. Thus, it becomes possible not only to economize resources and to improve practical efficiencies but also to develop a large-scale software system due to the accommodation of programs and data. Further, there is no priority in using the transmission line among the users or personal stations in such a communication system. Accordingly, there is no master/slave order or the like which is often provided in other systems, so that communication is allowed with equality between any of the stations connected to the transmission line. Further, since the transmission line, such as a coaxial cable, is constituted by a completely passive circuit, a highly reliable system may easily be provided.
Thus, this communication system has various advantages, but there is a possibility in this system that packet signals will collide with each other on the same transmission line since each station can begin transmitting data at any desired time. Such collisions between packet signals will be largely increased as the operation efficiency of the transmission line becomes higher.
To solve this problem, there have been proposed signal transmission systems such as the so-called "Priority Ethernet" and the so-called "Reservation Ethernet." In the former system, the priority of signal transmissions of each station is described in the preamble portion of a packet signal so that if a collision occurs between packet signals from different stations, that one of the packet signals having the higher priority is allowed to be transmitted preferentially. In the latter system, however, a master station which indicates an operational mode is always set so as to confirm whether each of the personal stations has a signal to be transmitted in the reserved mode, and the amount of information to be transmitted. According to the result of this confirmation, the master station determines, in every frame, the order of packets to be transmitted from the respective stations so as to allow signals to be transmitted in time division multiplex fashion in the transmission operation mode.
In the former proposed signal transmission system, however, there is still the problem of variations in signal transmission delay time due to a collision between packets having the same priority. Accordingly, this system is not suitable for real time transmission, such as conventional sound (e.g. telephone) communication, in which importance is attached to the real time correspondency between transmitting and receiving operations. In the latter signal transmission system, moreover, the abovementioned inter-station equality is lost because of the existence of the master station.
That is, if any failure occurs in the master station of this system, data communication is stopped, resulting in lowered reliability of the overall system.
In order to solve this problem, there has been proposed a digital signal transmission system in which real time transmission can be effected without losing the equality among personal stations. In this system, a frame which is cyclically repeated on the time axis is subdivided on the same time axis into a plurality of blocks so that each personal station may be given an opportunity of packet communication in one or more blocks. Thus, each station not only may have equality in using an empty block or blocks but also effect real time transmission, because an opportunity for signal transmission is periodically provided in every frame when the station occupies a certain block or blocks for a necessary period of time for signal transmission.
In a signal transmission system in which the above communication system is employed, there have been caused the following problems when a long communication cable is used:
(1) Distortion in the signal waveforms transmitted on the communication cable; and
(2) An increase in signal propagation delay time between stations.
Problem (1) is caused by the deterioration in the frequency characteristic of the communication cable in the high frequency band. If a high speed packet signal is transmitted through such a communication cable, the transmission waveform is distorted, causing a code error in the receiving station. There has been proposed, however, an effective solution to this problem.
FIG. 1 illustrates this solution. A plurality of stations are connected to a coaxial cable 2 which is in turn connected at its end to a terminator 1. The signal transmission system associated with the cable 2 is referred to as system #1. To expand the system, assume that another cable 4 is set up which is connected at its end to another terminator 3 and to which a plurality of stations 6.sub.1, . . . are connected respectively correspondingly through a plurality of taps 5.sub.1, . . . . The latter system is referred to as system #2. A repeating installation 7 is set up so as to interconnect the systems #1 and #2 to enable the two coaxial cables 2 and 4 to be equivalently regarded as a single coaxial cable, and a system #1 tap 8.sub.M and a system #2 tap 5.sub.M are connected through the repeating installation 7. In the repeating installation 7, there are provided transceivers 71, 72 and a repeater 73 for repeating signals transmitted through systems #1 and #2. The repeater 73 serves not only to cause signals transmitted on the coaxial cables 2 and 4 to match and to transfer a packet signal collision status, when it occurs, to the respective stations, but also to amplify the packet signal to correct the waveform distortion or shape the waveform. Thus, even if the coaxial cable is substantially long and a packet signal is transmitted along a long path, it is possible to prevent code errors from occuring.
With respect to the increase of the inter-station signal propagation delay time (problem (2) above), in the proposed communication system, as described above, a frame is subdivided into a plurality of blocks so that an opportunity of packet communication by blocks is given to each station. In this case, however, the timing of the initiation of packet signal transmission becomes a problem. For example, as shown in FIG. 2, assume that a station C is located at a longitudinally central position of a first system's coaxial cable 2, which is connected at its opposite ends to impedance matching terminators 1.sub.1 and 1.sub.2. Another station S is located between the station C and the terminator 1.sub.1 and is now effecting a signal transmitting operation. In this case, the packet signal sent out from the station S may be received by the station C and further stations R.sub.1 to R.sub.4 on the coaxial cable 2 at different points of time depending on the signal propagation delay time on the cable. Accordingly, if each station sends out its own signal without considering this delay time, there will be a possibility of the occurrence of a condition such that adjacent packets on the coaxial cable may overlap with each other.
To prevent such a serious condition from occurring, the concept of a guard time .tau.g is used in this system. That is, an empty bit string, which is called the guard time .tau.g, is positioned between the respective packets to thereby prevent a collision between the packets. To this end, signal transmission from each station is controlled such that the packet signals transmitted from the respective stations may be equidistantly spaced in a row at the receiving position of the station C, and the guard time .tau.g is set so as to be equal to or more than two times the signal propagation delay time between the station C and the furthermost station.
That is, it is necessary to set the length of the guard time .tau.g proportionately to the length of the communication cable. Therefore, if the cable is long, the transmission efficiency will be lowered. Further, it is necessary to change the guard time .tau.g in all of the previously set up stations when the system is expanded or when a plurality of systems are interconnected, as for example shown in FIG. 1.
In sum, when it is intended to effect system expansion or interconnection among a plurality of systems in a communication system using the above proposed digital signal transmission system, there have been limitations to the extent and scale thereof.